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'''[wiki:GraspSwControllerCmd readcal]''' |
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[[TracNav(GraspContents)]] |
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||Command:||'''readcal'''|| |
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||Contexts:||Network socket, serial console, stage2 only|| |
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||Function:||Calibrate signal chain to bring in range of the ADCs.|| |
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||Required Parameters:||None.|| |
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||Optional Parameters:||If '''adulevel''' omitted, readcal just measures pedestal.|| |
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||'''adulevel='''||A number of counts, like '''5000'''|| |
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||'''channel='''||[r] Select between '''r'''ed '''g'''reen and '''b'''lue ADC channel|| |
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||'''cellrow='''||(Use [wiki:GraspSwControllerCmdOtacal otacal] command instead of readcal for OTAs)|| |
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||'''dev='''||[current] Select dev '''0''' or '''1''' (>= r5696 stage2 ONLY)|| |
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[[Image(Offsetting_CAL_STARGRASP_CCD_case.png)]] |
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== What Does "readcal" Do? == |
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Each pixel value generated by the STARGRASP controller is the |
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result of multiple individual digital samples of the channel's |
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internal Analog to Digital Converter (ADC) 0 to 4V range, shown above. The process used |
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is known as Digital Correlated Double Sampling (DCDS). Although |
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multi-sampling readout parameters are configurable at runtime with the '''cestlavie''' program and the '''[wiki:GraspSwControllerCmdClvset clvset]''' command, most STARGRASP CCD |
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readouts sample the reference pedestal level (ped) for each pixel |
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four times, and the video signal level (sig) four times. All eight of these |
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samples must be within the 0-4V limits of the 16-bit ADC (0..65535). |
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The difference between the average (arithmetic mean) of the |
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four reference pedestal values is subtracted from the average |
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of the video to produce the pixel value. |
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All four of the reference pedestal samples must always be above |
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the floor that the A/D can measure, and an internal offset voltage |
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(per channel) must be used to "steer" the value to something |
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above zero, but not too high. (The side effect of being too |
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high would be reduced dynamic range.) |
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In order to converge more quickly, the starting point for the readcal algorithm is the last known |
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calibration value. If it fails to converge, that offset voltage |
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(and next starting point) is set to 0 uV. For this reason, |
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sometimes readcal must be run twice to succeed, especially if |
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incorrect values (from a different boardset or preamp, for example) |
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were used as the starting point. |
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If readcal is taking a long time to complete, or of it is failing to converge, it is often because a new |
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starting point should be recorded. The other common cause of failure can be super-saturation of the CCD. |
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The CCD should always be dark (shutter closed, no major light leaks) and be cleaned multiple times |
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before attempting readcal. |
